Solid solution in plumbous potassium oxysilicate affected by interaction of a lone pair with bond pairs

Abstract

During explorations of the chemical crystallography of lead silicates, the synthetic phase K₂Pb₄Si₈O₂₁ (here KPS), attracted our attention. Clear crystals when ground under acetone produced fine needles and fluffy aggregates which resembled chrysotile asbestos and had elicited some earlier interest¹. Evidence indicated tetragonal symmetry and despite several attempts no report of a successful structure solution has appeared. Indeed, the structure analysis proved to be a challenge, and revealed ∞¹[Si₈O₂₀] tubular columns and [Pb₂OPb₄O₂₄] clusters, the latter consisting of six Pb²⁺ cations situated on the vertices of a compressed octahedron. A novel feature is the occurrence of disordered (Pb²⁺, K⁺) as a ‘solid solution’, the Pb–K = 0.59 Å separation being a consequence of the 6s² Pb²⁺ lone pair acting on circumjacent bond pairs. In our initial experience, best convergences yielded the reliability index R ∼0.20. Unit cell parameters, space group and other structure cell criteria in Table 1 show good agreement between the present study and an earlier report₂ on KPS and isostructural Pb₂Pb₄Si₆Al₂O₂₁.